The present invention relates to an ultrasonic diagnostic apparatus, or in particular to a technique effectively applicable to the correction of the delay time distribution of the received signals of a probe attributable to the fact that the interior of a subject is an ununiform medium.
In the conventional ultrasonic diagnostic apparatus, an ultrasonic wave is transmitted to a subject from an ultrasonic probe including an arrangement of a plurality of ultrasonic transducers (elements), the reflected wave thereof is received and given a delay time distribution as a received wave signal thereby to generate an ultrasonic beam having a directivity in a predetermined direction, thus producing a tomogram of the subject. The ultrasonic diagnostic apparatus, which is free of a radiation exposure unlike the other diagnostic apparatuses and can clearly plot the flesh easily without using a contrast medium as described above, is an indispensable diagnostic apparatus in wide fields of applications. As compared with the X-ray apparatus, however, the resolution of the ultrasonic diagnostic apparatus is still low and is expected to be improved further. In a technique for improving the resolution of the ultrasonic diagnostic apparatus, the amount of the phase shift caused by the ununiform medium in the subject is measured, and based on this phase shift amount, the delay time distribution applied to the received wave signals is corrected, and then the received wave signals corresponding to the respective ultrasonic transducers are added.
First, FIG. 9 shows a diagram for explaining the delay time distribution for forming an ultrasonic beam of high resolution in a human body constituting an ununiform medium. The correction of the pulse wave front in the ununiform medium will be explained with reference to FIG. 9. In the description that follows, only the operation at the time of receiving the ultrasonic wave will be explained for simplicity""s sake.
In the case of a uniform medium with a known sound velocity, the wave front (the wave front of the received wave signal) of the pulse reflected from a reflector 906 reaches transducers 901 to 905 as an ideal wave front. In the process, due to the relative positions of the reflector 906 and the transducers 901 to 905, the reflected pulse reaches the transducer 903 earliest and the transducers 901, 905 latest. In order to assure the same arrival time of all the reflected pulses, the pulses received by the transducers 902 to 904 are given an appropriate delay. As a result, the same arrival time is secured for all the reflected pulses, and by the subsequent addition thereof, only the pulses received from the intended direction are amplified thereby to form a tomogram of a high resolution. In the case of a uniform medium of a known sound velocity, the delay to be given could be analytically determined as described below.
Assume that the distance between the transducers 901 to 905 and the reflector 906 is Li (1xe2x89xa6ixe2x89xa65), the initial sound velocity of the ultrasonic diagnostic apparatus is c, the delay time given to the received wave signals of the transducers 901 to 905 is xcfx84i (1xe2x89xa6ixe2x89xa65), and the maximum value of Li (1xe2x89xa6ixe2x89xa65) is Lmax. Then, xcfx84i can be expressed by equation 1 below.
xcfx84i=(Lmaxxe2x88x92Li)/cxe2x80x83xe2x80x83(1) 
Actually, however, an ununiform medium 907 exists between the transducers 901 to 905 and the reflector 906, and therefore the pulse wave front assumes a distorted wave front 908. As a result, although xcfx84i is optimum as an initial delay time given to the received wave signal of each transducer, the initial delay time is required to be corrected by an amount taking the distorted wave front 908 into consideration in order to produce a tomogram of a high resolution.
A technique for measuring this correction amount is described in xe2x80x9cIEEE Transactions on Ultrasonic, Ferroelectrics and Frequency Control, Vol. 39, No. 6, pp. 700-707, 1992 (hereinafter referred to as xe2x80x9creference 1xe2x80x9d) or xe2x80x9cIEEE 1991 Ultrasonics Symposium Proceeding pp. 1189-1193, 1991xe2x80x9d (hereinafter referred to as xe2x80x9creference 2xe2x80x9d). The technique described in these references is for correcting the effect that is had on the pulse wave front by an object having a different sound velocity, i.e. an ununiform medium which may exist between transducers for transmitting and receiving an ultrasonic wave and a reflector for reflecting the ultrasonic wave. According to this correcting technique, first, the amount of the phase shift of the reflected pulses resulting from the deviation of propagation time of the ultrasonic wave due to the ununiform medium is calculated by the correlating operation between all adjacent transducers. Then, based on the result of this calculation, the delay time of delay means is corrected thereby to correct the distortion of the pulse wave front due to the propagation of the ultrasonic wave through the ununiform medium for an improved resolution of the ultrasonic image.
The phase shift amount is determined using correlators for detecting adjacent phase shift of the outputs of the delay means for delaying the received wave signals of the transducers, for example. As a method of measuring the phase shift amount using the correlators, a technique is described in JP-A-1-135333 (hereinafter referred to as xe2x80x9creference 3xe2x80x9d). According to the measuring technique described in reference 3, first, a delay time (initial delay time) is set as an initial value of delay means for giving a delay time distribution on the assumption that an human body is a uniform medium having a known sound velocity. Then, the delay process, i.e. the phasing of the received wave signals is carried out. After that, the phase shift amount between adjacent received wave signals after the delay process, i.e. the phase shift amount between the output signals of the delay means is calculated using correlators, and based on this arithmetic output, the phase shift amount for the initial delay time is corrected. In this way, the resolution of the ultrasonic wave is improved by correcting the phase shift caused by the ununiform medium in the human body.
As the result of studying the prior art described above, the present inventor has discovered the following problem points.
The conventional ultrasonic diagnostic apparatus poses the problem that as many correlators as the outputs of the delay means less 1, i.e. the number of the ultrasonic transducers less 1 are required for calculating the correlation between all the adjacent outputs of the delay means, resulting in a large circuit scale required for signal processing.
A technique for solving this problem is incorporated in an ultrasonic diagnostic apparatus described in JP-A-9-103429 (hereinafter referred to as xe2x80x9creference 4xe2x80x9d) filed by the same applicant. The ultrasonic diagnostic apparatus described in reference 4 comprises delay means for giving a different delay time distribution for each of the received wave signals output from ultrasonic transducers, first adding means for reducing the number of signals by adding the received wave signals output from the delay means, i.e. the received wave signals after phasing, second adding means for generating a single ultrasonic beam by adding the received wave signals output from the first adding means, and correlating means for performing the correlation calculation of adjacent output signals (adjacent received wave signals) from the received wave signals output from the first adding means and measuring the phase shift amount between the adjacent output signals. This ultrasonic diagnostic apparatus is so configured as to comprise Na delay means corresponding to Na ultrasonic transducers, for example, and the first adding means adds each two or more of the Na adjacent inputs to reduce the number of the outputs to Nb. In the ultrasonic diagnostic apparatus described in reference 4, therefore, correlating means can be configured with Nb-1 correlators corresponding to the first adding means so that the correction amount of the initial delay time, i.e. the phase shift amount can be measured with a small circuit scale.
With the ultrasonic diagnostic apparatus described in reference 4, the phase shift amount calculated in each correlator fails to coincide with the number of delay means, and therefore means is required to set a correction amount given to each delay means from the phase shift amount detected by the correlating means. Reference 4, however, includes no description of a method of calculating the correction amount given to each delay means nor a configuration for obviating the inconvenience, and therefore poses the problem that the phase shift amount detected by each correlator cannot be reflected in each delay means.
An object of the present invention is to provide a technique for making it possible to reduce the distortion of the received wave signal due to the ununiformity in an human body accurately with a small circuit scale.
Another object of the invention is to provide a technique for making it possible to improve the resolution of an ultrasonic image.
Still another object of the invention is to provide a technique for making it possible to improve the efficiency of diagnosis of a subject.
The above and other objects and the novel features of the invention will be made apparent by the description of the present specification and the accompanying drawings.
Representative features of the invention disclosed by this application will be briefly explained below.
(1) An ultrasonic diagnostic apparatus comprises a probe including an arrangement of elements for transmitting and receiving ultrasonic pulses to and from the interior of a subject, delay means for delaying the received wave signal from each element of the probe, and a plurality of adding means connected in series to the output of the delay means for adding the output signal of the preceding stage thereby to form an ultrasonic beam, the apparatus further comprising correlating means for performing the correlation calculation of two or more output signals of any of the adding means and delay correcting means for estimating as many delay correction amounts as the input signals to the delay means from the time difference between adjacent signals generated from the correlating means and correcting the delay time of the received wave signals.
(2) The ultrasonic diagnostic apparatus as described in (1) above, wherein said delay correcting means includes linear arithmetic means.
(3) The ultrasonic diagnostic apparatus as described in (1) or (2) above, wherein the number of the input signals to the delay means is a multiple equal to the power of 2 of the input signals to the correlating means.
(4) The ultrasonic diagnostic apparatus as described in any one of (1) to (3) above, wherein the delay correcting means estimates the delay correction amount of the elements other than one of the outermost elements with reference to the delay time for said one of the outermost elements.
According to the means of (1) to (4) described above, the correlating means calculates the time difference between adjacent signals by the correlating operation of two or more output signals of the adding means, and based on this time difference, the delay correcting means estimates as many delay correction amounts as the input signals to the delay means, and the delay means corrects the delay time of the received wave signals based on the delay correction amounts. In this way, the number of the time differences calculated by the correlating means, i.e. the number of the adjacent signals input to the correlating means can be reduced, and therefore the circuit scale of the correlating means can be reduced. In the process, by estimating the delay correction amount for each ultrasonic transducer from the time difference between the adjacent signals by the delay correcting means after addition, the sound velocity information loss by the addition for each ultrasonic transducer can be restored. Therefore, the distortion of the received wave signals, i.e. the phase shift of the received wave signals caused by the ununiformity in the subject can be eliminated accurately with a small circuit scale. As a result, the distortion of the ultrasonic beam output from the adding means can be reduced, thereby making it possible to improve the resolution of the ultrasonic image. Consequently, the S/N of the ultrasonic image can be improved. Thus, the diagnosis efficiency can be improved.
At the same time, the circuit configuration can be simplified by configuring the delay correcting means with a linear arithmetic operation, and therefore a fast arithmetic operation is made possible. Also, the circuit scale of the delay correcting means can be reduced, and therefore the ultrasonic diagnostic apparatus can be reduced in size.